The present invention relates to solder joints between silicon members and members comprising a refractory metal, and a process for forming such joints. The invention may be employed, for example, in the manufacture of semiconductor devices.
In the manufacture of semiconductor devices, particularly high-power semiconductor devices, it is often desired to connect silicon semiconductor material to a heat sink of a high conductivity metal, for example copper. To help relieve thermal stress which may occur at such a connection it is conventional to join an intermediate member comprising a refractory metal, for example molybdenum, between the silicon semiconductor material and the high conductivity metal heat sink. Conventionally, the silicon and the refractory metal members are disc-shaped.
Aluminium has long been used as the solder metal for joining silicon and molybdenum discs together, the soldering being accomplished somewhat above the eutectic temperature of aluminium and silicon (577.degree. C.). The aluminium may initially contain a proportion of silicon, typically the eutectic composition, 11.7% Si by weight, although pure aluminium may be used. In each case, material is normally dissolved from the surface of the silicon disc during the soldering process, although more is dissolved if pure aluminium is used as the solder than if the eutectic composition is used.
Another effect involves a reaction which occurs between the silicon content of the aluminium-silicon alloy and the molybdenum surface, and results in the formation of an interfacial layer of molybdenum disilicide. This denudes the aluminium-silicon alloy of some of its silicon content at the interface and the consequent concentration gradient causes transport of silicon from the silicon disc surface towards the molybdenum surface. Such further dissolution of the surface of the silicon disc is generally undesirable, especially as there is often a tendency for it to occur non-uniformly, resulting in stepped or spiked erosion of the silicon surface.
Further, aluminium, being a dopant in silicon, forms a specifically p-type contact with the silicon unless the total concentration of n-type counter-dopants (e.g. phosphorus) is sufficient to maintain approximately ohmic properties at the contact, through degeneracy of the silicon electronic band structure. This requirement imposes a combination of constraints on permissible dopant profiles and on alloying erosion (dissolution) effects, as described above, at the silicon to aluminium-silicon boundary. In particular, where the silicon dopant distribution includes small and critical features, it is vital that these shall not be lost by dissolution into the aluminium-silicon solder at the site of an irregularity (spike) in the alloying front.
FIG. 1 shows a typical section through a joint which has been formed using such a prior art method. A silicon disc 1 is shown attached to a molybdenum disc 2 by a layer of aluminium-silicon solder 3. It can be seen that the penetration of the solder layer 3 into the diffused silicon disc 1, is irregular as shown by reference numeral 4. In some small areas, the depth of penetration caused by the dissolution of silicon may cause diffused-in features in the silicon disc, such as n.sup.+ type region 5, to be seriously eroded or even annihilated, with the most undesirable consequences. A layer of molybdenum disilicide 6 several microns in thickness forms between the solder 3 and the molybdenum disc 2.
A variety of techniques have been proposed to improve uniformity and reduce the silicon erosion at such soldering joints, for example producing the required bonding at the lowest possible temperature or coating the surface of the molybdenum component to inhibit the formation of molybdenum disilicides. Such techniques have often been less successful than might have been hoped. Other methods of forming the bond, such as diffusion soldering [Jacobson, D.M. and Humpston, G., High Power Devices: Fabrication Technology and Developments, Metals and Materials, December 1991] claim success but at the cost of some complexity.
An alternative approach, that of forming a barrier layer at the silicon surface in order to prevent the dissolution or modification of the dopant structure within the silicon has been proposed, mainly in connection with thin deposition films and VLSI technology, e.g. Babcock, S.E. and Tu, K.N., Journal of Applied Physics, vol. 59, No. 5, pp 1599-1605, March 1986. The technique therein disclosed does not, however, extend to the necessary area nor does it provide for the attachment of a heavy molybdenum supporting electrode, such as would be required by a large area power device extending up to 100 mm or so in diameter.
As prior art, there may also be mentioned GB-A-2 238 267 which discloses a process for brazing a silicon body to a metal body, the silicon body being provided with an adherent oxide film. The oxide film is coated with a metal layer structure which provides a brazable surface. Attack of the silicon surface by braze alloy is prevented by the oxide film. The metal layer structure comprises titanium, molybdenum and nickel. The braze is a silver/copper alloy.